Since the initial publication of Bednorz and Mueller, "Possible High T.sub.c Superconductivity in the Ba-La-Cu-O System", Z. Phys. B--Condensed Matter 64, p. 189-193 (1986), investigation has revealed a large family of crystalline oxides which exhibit superconductivity at higher temperatures than previously thought possible. The terms "high temperature superconductor" and "high temperature superconductive" are herein employed to describe as a class those crystalline oxides that are capable of exhibiting superconductivity at temperatures greater than 30.degree. K.--i.e., T.sub.o &gt;30.degree. K. The term "T.sub.o " is employed to indicate the superconductivity onset temperature--i.e., the highest temperature at which superconductivity is observed. The term "T.sub.c " is employed to indicate the onset critical temperature, the temperature at which a transition to superconductivity begins.
H. Maeda, Y. Tanaka, M. Fukutomi, and Y. Asano, "A New High T.sub.c Superconductor Without a Rare Earth Element", Japanese Journal of Applied Physics, Vol 27, No. 2, pp. L209 & L210, first reported that at least one compound of bismuth, strontium, calcium, copper, and oxygen had been found to be superconducting.
One of the difficulties that has arisen in attempting to form films of bismuth mixed alkaline earth copper oxides that are superconducting at the highest attainable temperatures is that the conductive films are comprised of a mixture of different crystalline phases. Investigations have suggested that there are at least two bismuth mixed alkaline earth copper oxide crystalline phases present both having a pseudotetragonal crystal structure with crystal cell a and b axes of 5.4 .ANG., but differing in the length of their crystal cell c axes.
The less desirable of these phases, exhibiting a T.sub.c of 85.degree. K., appears to be formed of an oxide containing bismuth strontium calcium and copper in the proportions Bi.sub.2 Sr.sub.2 CaCu.sub.2. This lower onset T.sub.c superconductive phase is also referred to as the BSCCO-2212 phase. This crystalline phase can be identified by a crystal cell 30.7 .ANG. c axis and an X-ray diffraction angle of 5.8.degree. for the (002) line.
A more desirable phase, exhibiting a higher T.sub.c of 110.degree. K., appears to be formed of an oxide containing bismuth strontium calcium and copper in the proportions Bi.sub.2 Sr.sub.2 Ca.sub.2 Cu.sub.3. This higher T.sub.c phase is also referred to as the BSCCO-2223 phase. This crystalline phase can be identified by a crystal cell 37 .ANG. c axis and an X-ray diffraction angle of 4.9.degree. for the (002) line.
Lelental and Romanofsky U.S. Ser. No. 546,458, filed Jun. 29, 1989, titled PROCESSES OF FORMING CONDUCTIVE FILMS AND ARTICLES SO PRODUCED, commonly assigned, discloses that the proportion of the BSCCO-2223 phase in a thin film can be increased by satisfying the metal ratio: EQU M:Cu is&gt;1.00
where
M is bismuth and 0 to 50 percent lead, based on bismuth and lead combined. The possibility of coating on a silver or gold barrier layer is disclosed.
Mir et al U.S. Pat. No. 4,988,674 discloses a process of forming a flexible article by coating a release layer of gold, silver or a platinum metal on a refractory substrate and overcoating the release layer with a conductive cuprate layer. After the conductive cuprate layer has been formed, a protective layer can be coated over it, and subsequently the protective layer, conductive cuprate layer and release layer can be stripped from the refractory substrate to produce a flexible electrical conductor.
Hakuraku et al, "Superconducting Thin Films of Bi-Pb-Sr-Ca-Cu-O by DC Magnetron Sputtering from a Single Target", Japanese Journal of Applied Physics, Vol. 27, No. 11, Nov. 1988, pp. L2091-L2093, reports successfully preparing a 110.degree. K. T.sub.c phase containing thin film on a magnesia substrate using a target consisting of bismuth, lead, strontium, calcium, copper and oxygen.
Hoshino et al, "Preparation of Superconducting Bi-Sr-Ca-Cu-O Printed Thick Films on MgO Substrate and Ag Metal Tape", Japanese Journal of Applied Physics, Vol. 27, No. 7, July, 1988, pp. L1297-299, reports preparing thick films on a silver substrate surface having zero resistance at 76.degree. K. using powders with a nominal ratio of Bi:Sr:Ca:Cu of 1:1:1:2.
Shimojima et al, "Preparation of High-Tc Superconductive Bi-Pb-Sr-Ca-Cu-O Film by Pyrolysis of Organic Acid Salts," Japanese Journal of Applied Physics, Vol. 28, No. 2, Feb. 1989, pp. L-226-228, reports forming a high Tc phase (a 2223 phase) by employing a ratio of Bi:Pb:Sr:Ca:Cu of 0.7:0.3:1.0:1.0:1.8.